sodium hypochlorite as dentin pretreatment for

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Alle Rechte vorbehalten

Since Buonocores attempt1 to bond acrylic resin to enam-el using phosphoric acid, searching for an adhesive ma-terial that bonds effectively to hard tooth structure has beenthe purpose of the extensive research done in the followingyears. The exchange process that involves the replacementof minerals removed from the hard dental tissues by resinmonomers is the essential mechanism of bonding to enam-el and dentin. These resin monomers, when set, become mi-cromechanically interlocked in the created porosities.5

Sodium Hypochlorite as Dentin Pretreatment for Etch-and-Rinse Single-bottle and Two-step Self-etching Adhesives: Atomic Force Microscope and Tensile BondStrength EvaluationAmr S. Fawzya/Mohamed A. Amerb/Farid S. El-Askaryc

Purpose: The aim of this study was to evaluate the effect of using 5.25% commercial sodium hypochlorite treatmentprior to the application of etch-and-rinse and self-etching adhesives on dentin surface microtopography and tensilebond strength.

Materials and Methods: Thirty-two noncarious, nonrestored human third molars were collected. The occlusal enamelof all teeth was removed using diamond disks to expose flat dentin surfaces. The exposed dentin surfaces wereabraded using 600-grit SiC disks, to create a uniform dentin smear layer. For AFM characterization, 12 teeth wereequally divided into 4 groups according to the proposed dentin surface treatment. Three dentin disks, 2 mm thick,were evaluated per group using tapping mode assessment. Twenty teeth were used for TBS and SEM evaluation andwere equally divided into 4 groups, according to the proposed dentin surface treatment. For TBS, 8 dentin/compositeslabs, 2 mm thick, were used in each group, while for SEM evaluation 2 slabs were used. Each slab was tested in ten-sion at a crosshead speed of 0.5 mm/min until failure occurred. The samples were examined using SEM operated at30 kv to evaluate the hybrid layer photographically at 1500X. Statistical analysis was carried out using StatsDirect2.5.7. One-way ANOVA followed by Tukey-Kramer multiple-comparison post-hoc tests were performed to test the differ-ence between roughness parameters and TBS between groups.

Results: Sodium hypochlorite followed by the application of 37% phosphoric acid significantly increased the arith-metic average of the absolute values of surface height deviations (Sa), the surface area ratio which expresses theratio between the surface area (taking the z height into account) and the area of the flat x,y plane (Sdr), and the sur-face bearing index (Sbi) parameters, while the application of sodium hypochlorite prior to the application of the self-etching primer significantly increased the valley fluid retention index (Svi) parameter. Self-etching primer withoutsodium hypochlorite pretreatment significantly increased the core fluid retention index (Sci) parameter. Sodiumhypochlorite/AdheSE (7.42 2.16 MPa) significantly increased TBS value compared to other groups. However, no sta-tistically significant difference was found between sodium hypochlorite/Excite (4.68 1.26 MPa), AdheSE (4.42 1.36), and Excite (4.06 1.35). Remnants of smear layer were detected with areas devoid of resin tags in SEM im-ages of samples bonded with AdheSE self-etching adhesive, in contrast to samples bonded with sodium hypochloritefollowed by AdheSE self-etching adhesive.

Conclusion: The application of 5.25% commercial sodium hypochlorite with rubbing action for 60 s (total applicationtime 120 s) seems to positively influence the TBS of the self-etching adhesive; however, it has no significant effect onTBS of etch-and-rinse single-bottle adhesive to dentin. The addition of functional roughness parameters to study thedentin surface was shown to be of importance in evaluating the relationship between bond strength and surfacetopography of conditioned dentin.

Keywords: sodium hypochlorite, dentin adhesives, AFM, tensile bond strength.

J Adhes Dent 2008; 10: 135-144. Submitted for publication: 06.01.07; accepted for publication: 01.05.07.

Vol 10, No 2, 2008 135

a Lecturer, Dental Biomaterials Department, Faculty of Dentistry, Ain-ShamsUniversity, Cairo, Egypt.

b Associate Professor, Engineering and Surface Metrology Department, Na-tional Institute of Standards, Cairo, Egypt.

c Associate Professor, Operative Dentistry Department, Faculty of Dentistry, AinShams University, Cairo, Egypt.

Correspondence: Dr. Farid S. El-Askary, Operative Dentistry, Department ofConservative Dentistry, Faculty of Dentistry, Ain-Shams University, Cairo, Egypt.Tel: +202-419-6956, Fax: +202-682-0191. e-mail: [email protected]

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Alle Rechte vorbehaltenDentin bonding or adhesion refers to the micromechanicalcoupling or union of restorative materials to dentin, particu-larly dental composites, via an intermediary adhesive resinlayer.6 The use of acid etching as a dentin surface treat-ment was reported as a step to improve adhesion for a vari-ety of procedures in restorative and preventive dentistry.30Although dry bonding is a clinically acceptable technique,39wet bonding is recommended to improve the bond strengthof the total-etch adhesives to dentin.23

Self-etching adhesives were introduced on the market inorder to simplify the bonding steps, thus reducing the actu-al bonding time. Since etching and priming of the dentin sur-face is done in the same step,12 the resin monomers pene-trate the whole depth of the demineralized dentin. Incom-plete resin penetration to this depth will leave an exposeddemineralized dentin zone at the base of the hybrid layer. Inorder to increase the bond longevity for the adhesive restora-tions, it is very important to eliminate this demineralizeddentin zone from the bond structures.10

Many studies4,16,26,31,32,34,36,42,43 have evaluated the ef-fect of sodium hypochlorite treatment on the performanceof the different adhesives to dentin. It was found that sodi-um hypochlorite application either had no effect,4,16,32,42 orpositively31 or negatively affected16,26,31,34,36,43 the perfor-mance of the different dentin adhesives. The use of sodiumhypochlorite after etching of the dentin surface was alsofound to remove the exposed collagen fibers that altered thedentin surface characterization.3,20,39

The advantage of the tensile bond strength (TBS) testingmethod is that it provides an estimation of the possible clin-ical performance of dentin adhesives. The data obtainedfrom this test were shown to be more useful than previousmethods for the assessment of the bond strength todentin.21 Burrow et al2 studied the TBS of several adhesivesto dentin and reported that none of the adhesives testedreached the 20 MPa suggested for producing a gap-freerestoration. In addition, they found that it was not essentialfor the bond strength to reach the suggested 20 MPa.

AFM has been applied in dental biomaterial sciences particularly in mineralized tissue research, to characterizemineralized dental tissues and the effects of deminera-lization and surface modification procedures.15 Many authors8,11,17,24,25 suggested various important contribu-tions by AFM for the investigation of dentin surfaces. Theaim of their work was to look at the surface structure ofetched dentin, the mechanical properties of sound and de-mineralized dentin, and the properties of the interface be-tween different adhesive materials and dental hard tissues. The aim of this study was to investigate the influence of5.25% commercial sodium hypochlorite applied for 120 s asa dentin pretreatment on changing the surface microtopo-graphy and TBS before the application of a single bottleetch-and-rinse and a two-step self-etching adhesive system.

MATERIALS AND METHODS

A total of 32 noncarious, nonrestored human third molarswas used. After extraction, the teeth were cleaned of bloodand soft tissue under running water and stored in distilled

water for no more than one month. The roots were embed-ded in self-cured acrylic resin 3 mm below the cemento-enamel junction. For sample preparation, the occlusalenamel was removed using a low-speed diamond disk in astraight handpiece fixed to a milling machine (Nouvage AG,Frsgert AF 30, Switzerland) under copious water spray toexpose flat dentin surfaces. The teeth were stored in distilledwater until further use.

AFM Surface CharacterizationTwelve teeth were used for AFM surface characterization. A2-mm dentin disk was prepared from each tooth using a dia-mond disk mounted to a milling machine. The upper surfaceof the dentin disk was perpendicular to the long axis of thetooth. The exposed dentin surfaces were wet abraded using600-grit silicon carbide paper (SiC) to create a uniform dentinsmear layer. The prepared disks were equally divided intofour groups according to the proposed dentin surface treat-ment. Materials, compositions (lot #), and manufacturers arepresented in Table 1. In the group treated with phosphoricacid (PHA), the dentin disks were etched with 37% phos-phoric acid gel for 15 s. The surface was rinsed with air/wa-ter spray for 15 s and dried using a cotton pellet (wet bond-ing). In the group treated with self-etching primer (SEP), Ad-heSE self-etching primer was applied over the dentin sur-faces, left for 15 s, then gently rubbed for another 15 s, andexcess solvent was removed using oil-free compressed air for3 s. For the group treated with sodium hypochlorite followedby phosphoric acid (SHC-PHA), two drops of 5.25% sodiumhypochlorite was applied over each dentin surface, gentlyrubbed for 60 s using a Vivadent microbrush applicator andleft for another 60 s, with a total treatment time of 120 s. Thedentin surface was washed for 60 s and air dried with oil-freecompressed air for 3 s. Following the application of the sodi-um hypochlorite, each dentin surface was etched with 37%phosphoric acid as in group PHA. In the group treated withsodium hypochlorite followed by self-etching primer (SHC-SEP), 5.25% sodium hypochlorite was applied to each dentinsurface as in group SHC-PHA, followed by the application ofAdheSE self-etching primer as described in group SEP.

Tapping mode measurements were performed with anAFM (Autoprobe CP-II, Veeco Autoprobe; Camarillo, CA, USA).The tapping mode operates by scanning a tip, attached tothe end of a vibrating cantilever, across the sample surface,so that it is in intermittent contact with the surface. The can-tilever amplitude is maintained constant by altering the ver-tical position of the piezoelectric scanner. For the tappingmode, a gold-coated all-silicon cantilever (Ultralevers, Veeco)with integrated high-aspect ratio conical tips was used. Thetypical radius of curvature of the scanning tip was 100A. Im-ages were recorded with a slow scan rate (1 Hz), and a res-olution of 512 x 512 pixels per image was chosen. Eachdentin disk was scanned at the central area three times. Thescanning area was 10 x 10 m2. The collected 3-D topo-graphical data were analyzed using data analysis software.For each group, the following amplitude, hybrid and func-tional roughness parameters were calculated: Sa: the arith-metic average of the absolute values of surface height devi-ations recorded within the sampling area; Sdr: the surfacesarea ratio which expresses the ratio between the surface

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area (taking the z height into account) and the area of theflat x,y plane; Sbi: the surface bearing index; Sci: the core flu-id retention index; and Svi: the valley fluid retention index.

Bonding ProceduresThe exposed dentin surfaces of the remaining 20 teeth werewet abraded as described previously for the specimens usedin AFM characterization. Two adhesive systems, one etch-and-rinse single-bottle (Excite) and one self-etching adhesive(AdheSE), were used according to the respective manufac-turers instructions.

The teeth were divided into 4 equal groups. In the grouptreated with Excite, the single-bottle adhesive (Ex), eachdentin surface was etched using 37% phosphoric acid gel for15 s. The surface was rinsed with air/water spray for 15 sand dried using a cotton pellet (wet bonding). One coat of Ex-cite adhesive was applied over the etched dentin surfaceand gently agitated for at least 10 s, excess solvent was re-moved using a gentle stream of air for 3 s, and light curingwas performed for 20 s using a light-curing unit (Cromalux-E, Meca-Physik Dental Division; Rastatt, Germany), with alight output of 600 mW/cm2. A 6-mm-high resin compositecrown (Tetric Ceram) was formed for each tooth. The resincomposite was applied in four increments, approximately1.5 mm thick each. Each increment was light cured for 40s. In the group treated with AdheSE, self-etching adhesive(Ad), AdheSE self-etching primer was applied over the dentinsurface, left for 15 s, gently rubbed for another 15 s, and ex-cess solvent was removed using oil-free compressed air for3 s. The adhesive was applied and left for 10 s, air thinned,and light cured for 20 s. Resin composite crowns wereformed as for group Ex. In groups treated with sodiumhypochlorite followed by AdheSE (self-etching adhesive

[H/Ad]) or treated with sodium hypochlorite followed by Ex-cite (etch-and-rinse single-bottle adhesive [H/Ex]), two dropsof 5.25% sodium hypochlorite was applied over each dentinsurface, gently rubbed for 60 s using a Vivadent microbrushapplicator, and left for another 60s (with total applicationtime 120 s). The dentin surface was rinsed for 60 s and airdried with oil-free compressed air for 3 s. Following the ap-plication of the sodium hypochlorite, Excite and AdheSEwere applied as described in Ex and Ad groups respectively.Resin composite crowns were formed as in group Ex.

The teeth with their attached composite crowns werestored in distilled water for 24 h to permit the termination ofthe polymerization process.29

Tensile Bond Strength TestingBefore preparing the composite/dentin slabs for TBS testingand SEM evaluation, each composite crown was measuredmesiodistally using a digital caliper. Ten 2-mm-thick slabswere prepared (5 teeth) for each group. From the central re-gion of each crown, two slabs were prepared. The slabs wereprepared using the same low-speed diamond disk. The spec-imens were vertically sectioned in a buccolingual direction.A fine high-speed diamond stone (Mani Dia-Burs, Tc-21,Takanezawa-mashi; Tochigi-Ken, Japan) with air/water spraywas used for trimming the slabs into an hourglass shapewith a cross-sectional area of approximately 6 mm2 0.05at the bonded interface (Fig 1a). No premature debondingoccurred during specimen preparation. Eight slabs wereused for TBS testing, while the other two slabs were pre-pared for SEM evaluation.

Each slab was fixed to a modified disposable acrylic resinjig (Fig 1c) for TBS testing. The dimensions of the acrylicresin jig were modified from that created by Nakabayashi et

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Table 1 Materials, compositions (lot #) and manufacturers used in this study

Material

Etch-and-rinse single-bottle system (Excite)

Two-step self-etching system (AdheSE)

Resin composite (Tetric Ceram)

Manufacturer

Ivoclar/Vivadent;Schaan/Liechtenstein

Ivoclar/Vivadent

Ivoclar/Vivadent

Composition

Acid etch: 37% phosphoric acid, thickening agent and pigments (lot #H01061)Adhesive: hydroxyethyl methacrylate (HEMA), dimethacrylate, phos-phoric acid acrylate, highly dispersed silicon dioxide, initiator and stabi-lizer in alcohol solution (lot # G14452)

Primer: dimethacrylate, phosphonic acid acrylate, initiator and stabi-lizer in aqueous solution (Lot # H27161)Adhesive: hydroxyethyl methacrylate (HEMA), dimethacrylate, silicondioxide, initiator and stabilizer (lot # H27162)

Resin: bis-GMA, urethane dimethacrylate (UDMA), triethylene glycoldimethacrylateFiller: barium glass, ytterbium trifluoride, barium-aluminum-fluorosili-cate glass, highly dispersed silicon dioxide and spheroid mixed oxide.Catalyst, stabilizer and pigments are additional contents (shade B3, lot# G27062)

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al.21 Two split copper molds were used to prepare the stan-dardized modified acrylic resin jigs (Fig 1b). Each mold wascomposed of two symmetrical halves. In the first mold, eachhalf had a dimension of 11 mm x 2 mm x 6 mm, with a cop-per pin 2 mm x 2 mm soldered to and extending from the bot-tom of the mold. In the second mold, each half had two lev-els with dimensions of 6 mm x 4 mm x 6 mm and 5 mm x 2mm x 6 mm, with a copper pin 4 mm x 2 mm soldered to andextending from the bottom of the mold. Slabs were fixed tothe acrylic jigs using cyanoacrylate adhesive. Two roundedL-shaped metallic pins 2 mm in diameter were attached tothe upper and lower jaws of a universal testing machine (LR5series, Lloyd Instruments; Fareham, UK). The assembly wasmounted through the opening created by the copper pins inthe acrylic jigs (Fig 1d). To prevent unnecessary torque onthe assembly, the openings of the acrylic jigs were slightlyenlarged before fixing the slab, to permit free movement ofthe assembly during its attachment to the metallic pins.Each slab was tested in tension at a crosshead speed of 0.5

mm/min until failure. The cross-sectional area of each frac-tured slab was confirmed using a digital caliper, and the re-spective load was divided by the cross-sectional area to cal-culate the TBS in MPa.

SEM EvaluationEach slab was polished using 600-grit silicon carbide paperdisks to produce smooth polished surfaces. The slab washeld at the resin composite, etched with 37% phosphoricacid gel for 15 s, rinsed for 15 s, and dried with compressedoil-free air. The slabs were immersed in 5.25% sodiumhypochlorite solution for 20 min, washed thoroughly underrunning water for 5 min, then immersed in ascending con-centrations of ethanol (50%, 70%, 90%) for 20 min each and100% for 1 h.

The samples were fixed with double-faced stickers on thespecimen holder and gold sputtered (S150A sputter coater,Edwards; London, UK) to render the samples electrically con-ducting, using a very thin layer of gold about 150 in thick-

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Fig 1c Schematic diagram of the modified acrylic jig.

Fig 1a Schematic diagram of tooth sectioning and the preparedsample.

Fig 1b Split copper molds.

a

c d

b

Fig 1d Schematicdiagram of thetested assembly.

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ness under vacuum (3x10-1 mbar) of inert gas (argon). Thesamples were examined using SEM (JEOL JXA-840A, Elec-tron Probe Micro-analyzer; Tokyo, Japan), operated at 30 kv.The resin/dentin interface was examined at 1500X.

All data were expressed as mean standard deviation(SD) and were analyzed using StatsDirect 2.5.7. One-wayanalysis of variance (ANOVA) followed by Tukey-Kramer mul-tiple comparison post-hoc tests were used to compare theroughness parameters and TBS between groups. Signifi-cance was set at p < 0.05.

RESULTS

AFM Surface CharacterizationAFM 3D images for dentin surfaces treated with phosphor-ic acid, self-etching primer, combined sodium hypochlor-ite/phosphoric acid and combined sodium hypochlorite/self-etching primer are shown in Figs 2a to 2d.

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Fig 2a AFM 3-D image (10 x 10 m) of dentin surfacestreated with 37% phosphoric acid (PHA). Arrowheads:open dentinal tubules; ID: intertubular dentin.

Fig 2c AFM 3-D image (10 x 10 m) of dentin surfacestreated with sodium hypochlorite followed by 37% phos-phoric acid (SHC-PHA). Arrowheads: open dentinaltubules; ID: intertubular dentin.

Fig 2d AFM 3-D image (10 x 10 m) of dentin surfacestreated with sodium hypochlorite followed by AdheSEself-etching primer (SHC-SEP). Arrowheads: open denti-nal tubules; ID: intertubular dentin.

Fig 2b AFM 3-D image (10 x 10 m) of dentin surfacestreated with AdheSE self-etching primer (SEP). S: smearlayer, arrows: smear plugs.

Figure 2a shows a dentin surface treated with 37% phos-phoric acid for 15 s. Open dentinal tubules (arrowheads) sur-rounded by intertubular dentin (ID) with no remnant ofsmear layer or plugs are evident. Figure 2b depicts a dentinsurface treated with AdheSE self-etching primer for 30 s. Re-maining smear layer (S) with smear plugs (arrows) occludingthe dentinal tubules can be observed. Figure 2c showsdentin surface treated with 5.25% sodium hypochlorite for120 s followed by 37% phosphoric acid for 15 s. Widely opendentinal tubules (arrowheads) with no smear layer remnantor smear plug are apparent. Figure 2d illustrates a dentinsurface treated with 5.25% sodium hypochlorite for 120 sfollowed by AdheSE self-etching primer for 30 s. Completedissolution of the smear layer and smear plugs is visible andthe dentinal tubules are widely opened (arrowheads).

Roughness ParametersResults of the statistical analysis of the calculated rough-ness parameters showed that sodium hypochlorite followed

I D

I DI D

SS

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by phosphoric-acid etching significantly increased Sa (363.4 17.08 nm), Sdr (25.44 0.61%) and Sbi (0.736 0.005)parameters among all tested groups. There was a statisticallysignificant difference between all tested groups for Sa andSdr parameters. There was no statistical difference betweengroups PHA and SHC-SEP (p = 0.2059) for the Sbi parame-ter; however, a statistically significant difference was foundbetween all other groups. Dentin treatment using SEP sig-

nificantly increased the Sci parameter compared to othergroups (1.82 0.059). There was a significant difference be-tween all tested groups in terms of the Sci parameter. Dentintreatment using SHC-SEP significantly increased the Svi pa-rameter compared to other groups (0.179 0.002). Therewas no statistical difference between groups PHA and SHC-PHA (p = 0.9752), while there was a statistically significantdifference among the rest of the tested groups (Figs 3a to 3e).

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SHC-PHA SHC-SEP PHA SEP

400

350

300

250

200

150

100

Sa parameter

(n

m)

Sdr parameter


(%

)

30

25

20

15

10

5

Sbi parameter


0.75

0.7

0.65

0.6

0.55

0.5

Sci Parameter


1.8

1.6

1.4

1.2

Svi Parameter


0.18

0.16

0.14

0.12

TBS

H/Ex H/Ad Ex Ad

(M

Pa)

10

8

6

4

2

Fig 3d Error bars of means and standard deviations of the Sciparameter.

Fig 3c Error bars of means and standard deviations of the Sbiparameter.

Fig 3a Error bars of means and standard deviations of the Saparameter.

Fig 3b Error bars of means and standard deviations of the Sdrparameter.

Fig 3e Error bars of means and standard deviations of the Sviparameter.

Fig 3f Error bars of means and standard deviations of TBS.

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Tensile Bond StrengthResults of the statistical analysis of the calculated tensilebond strength (Table 2 and Fig 3f) showed that the treatmentof dentin surfaces with 5.25% sodium hypochlorite followedby the application of the self-etching adhesive (group H/Ad)yielded the highest mean TBS value (7.42 2.16 MPa),which was statistically significant in all tested groups. Therewas no statistically significant difference between Ex, SEP,and H/Ex groups (4.06 1.35, 4.42 1.36, and 4.68 1.26, respectively).

SEM EvaluationFigures 4a to 4d show SEM photographs of resin/dentin in-terfaces treated with etch-and-rinse, self-etching, combinedsodium hypochlorite/etch-and-rinse and combined NaOCl/self-etching adhesives.

Figure 4a depicts the resin/dentin interface after treat-ment with the etch-and-rinse single-bottle adhesive (Excite).The hybrid layer is infiltrated with adhesive filler (between ar-rows). Numerous funnel-shaped resin tags (R) extending

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Fig 4a SEM of resin dentin interface for speci-mens bonded with Excite etch-and-rinse adhesive. Hybrid layer between arrows; A = adhesive layer, R = resin tags, D = dentin.

Fig 4b SEM of resin dentin interface for speci-mens bonded with AdheSE self-etching adhesive.Hybrid layer between arrows; A= adhesive layer, R = resin tags S = remnants of smear layer, and D = dentin.

Fig 4c SEM image of resin/dentin interface forspecimens bonded with sodium hypochlorite/Ex-cite etch-and-rinse adhesive. Hybrid layer betweenarrows, A = adhesive layer, R = resin tags, and D =dentin.

Fig 4d SEM of resin dentin interface for speci-mens bonded with sodium hypochlorite/ AdheSEself-etching adhesive. Hybrid layer between ar-rows; A = adhesive layer, R = resin tags, D =dentin.

Table 2 Means and standard deviations (SD) of TBS inMPa for all tested groups

Groups Means SD Tukey

Ex 4.06 1.35 BSEP 4.42 1.36 BH/SEP 7.42 2.16 AH/Ex 4.68 1.26 B

n = 8. Means standard deviations (MPa). Mean values with sameletters are not statistically significant at p 0.05.

A A

A A

R

R

R

D

D

R

R

S

D D

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Alle Rechte vorbehaltenfrom the hybrid layer are observed. Figure 4b shows theresin/dentin interface after treatment with the two-step self-etching adhesive (AdheSE). The smear layer is incorporatedwithin the hybrid layer (between arrows). Remnants of smearlayer (S) can be seen. Few resin tags and areas completelydevoid of resin tags (R) are observed. Figure 4c illustratesthe resin/dentin interface after treatment with 5.25% sodi-um hypochlorite for 120 s followed by the application of thetotal-etch single-bottle adhesive (Excite). Hybrid layer (be-tween arrows) is infiltrated with adhesive fillers, with longand short resin tags (R) extending from it. Figure 4d showsthe resin/dentin interface after treatment with sodiumhypochlorite for 120 s followed by the application of the two-step self-etching adhesive. Here there was complete disso-lution of the smear layer with a very thin hybrid layer, whichis not clearly seen (between arrows). Complete infiltration ofthe demineralized layer by numerous resin tags (R) is shown.

DISCUSSION

Bonding to dentin is achieved by the diffusion and subse-quent polymerization of the adhesive monomers into theporosities that were created after acid etching.5 The strengthand durability of the bond is a result of the interaction be-tween variations in resin diffusion through the demineral-ized dentin and the inherent properties of the polymer.13

Sodium hypochlorite has nonspecific proteolytic and dis-infectant properties.18 Because of these properties, it iswidely used in a variety of dental procedures, such as en-dodontic treatment, chemomechanical caries removal,3 anddentin bonding techniques.4,26,27,31,32,34,42,43

In this study, the combined use of both tapping-modeAFM to characterize the treated dentin surfaces and SEMimaging of thr resin/dentin interface could be of importancein understanding resin/dentin TBS results. AFM was chosento study the dentin surface changes as a result of the vari-ous treatments because of its ability to study surfaces in thefully hydrated state. It was also of particular importance instudying the surfaces with soft and weakly attached mate-rials.28 It provides images with high resolution without ex-posing the treated surfaces to the relatively damaging ef-fects of sample preparation for SEM.9,28 Although manyroughness parameters could have been calculated based onthe quantitative AFM 3D data, the authors of the currentstudy thought that the five selected parameters would be ofsignificant value in explaining the results recorded from TBStesting.

Jacques and Hebling12 reported that the application of amild conditioner, such as EDTA, improved the bond strengthof self-etching adhesives to dentin. They attributed the im-provement in bond strength to the removal of the smear lay-er, which prevents direct contact of the self-etching adhesivewith dentin. Consequently, removal of the smear layer facil-itates the formation of a stronger and more homogeneoushybrid layer.

Sodium hypochlorite was applied to the dentin surfacewith gentle rubbing action for 60 s to enhance the effect ofsodium hypochlorite in dissolving the smear layer. It was pre-viously reported that the rubbing of sodium hypochlorite for

120 s on the dentin surface effectively removed the smearlayer, opened the dentinal tubules, and increased inter-tubular roughness. In addition, the shear force during rub-bing action improved the proteolytic effect of sodiumhypochlorite.20 For these reasons, the combined chemome-chanical dissolution of smear layer with sodium hypochloritewas an objective in this study. Extending the rinsing time ofsodium hypochlorite up to 60 s might have prevented the for-mation of sodium chloride crystals that would have closedthe tubule orifices.20

It was reported that sodium hypochlorite does not influ-ence4 or significantly decreases34 the bond strength of two-step self-etching adhesive (SE Bond) to dentin, which is incontrast to our results. The difference in methodology,dentin substrate, and the type of adhesive used could be thereason for the dissimilar results. The increase in TBS valueof the combined sodium hypochlorite/AdheSE could be ex-plained by the following: first, AFM imaging (Fig 3d) showedcomplete dissolution of the smear layer and smear plugs.This was confirmed by SEM evaluation, which also detectedcomplete infiltration of the demineralized layer by numerousresin tags (Fig 4d). Although sodium hypochlorite facilitatedthe penetration of resin monomer, it appeared that not allexposed collagen was encapsulated in resin, resulting in thepresence of a gap between the hybrid layer and the under-lying mineralized dentin, which occurred during SEM prepa-ration. This explanation needs further investigation. Rubbing5.25% sodium hypochlorite for 60 s partially dissolved thesmear layer, thus facilitating complete dissolution of thesmear layer and smear plugs by AdheSE primer. Second, thesignificant increase in both the arithmetic average of the ab-solute values of surface height deviations from main plane,Sa (p = 0.0023), and the percentage increase in dentin sur-face area, Sdr (p < 0.0001) (Figs 3a and 3b, respectively)could favor the micromechanical interlocking and stress dis-tribution along the resin/dentin interface. Third, the appli-cation of sodium hypochlorite prior to the application of Ad-heSE significantly increased the surface bearing index, Sbi(p < 0.0001), and the void volume in the valley zone, Svi (p< 0.0001) (Figs 3c and 3e, respectively). Increasing surfacebearing ability and the void volume in the valley zone couldhave facilitated the flow of the relatively viscous adhesivemonomer into these spaces. Consequently, this might havedecreased the possibility of formation of air voids at theresin/dentin interface, which may have led to less stressconcentration, resulting in increased bond strength. Func-tional parameters describe the bearing and fluid retentionability on surfaces. Large Sbi values indicate a good surfacebearing property, large Svi values indicate large void vol-umes in the valley zone, while large Sci values indicate thatthe void volume in the core zone is large. For a surface hav-ing a Gaussian height distribution, Sbi, Svi, and Sci para-meters approach 0.608, 0.11, and 1.56, respectively.37

Unfortunately, bearing ability and fluid retention proper-ties of dentin surfaces have, to our knowledge, been ne-glected in resin/dentin bonding studies. However, wethought that these functional parameters could help de-scribe the ability of the dentin surface to retain adhesivemonomers and consequently the capacity of these mono-mers to penetrate the dentin substrate.

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Alle Rechte vorbehaltenRegardless of the increase in TBS value recorded in ourstudy, we cannot decisively discount the effect of the pres-ence of protein chloramine-derived radicals14 in sodiumhypochlorite-treated dentin, which could result in prematurechain termination and incomplete resin polymerization,16leading to reduction in bond strength. However, bondstrength is multifactorial in nature, having many variables af-fecting it. Therefore, further studies might be of importancein determining the effect of using sodium hypochlorite priorto the application of the different self-etching adhesives onthe market.

Etch-and-rinse single-component and self-etching adhe-sive systems tested in this study showed no significant dif-ference in TBS values. The obtained data is consistent withprevious studies,19,35 which reported that the dentin bondstrength of self-etching adhesives was comparable to that ofthe etch-and-rinse systems. One of the advantages of self-etching adhesives is that dentin conditioning and primingoccur simultaneously, resulting in the formation of a strongvoid-free hybrid layer.12 AdheSE is a self-etching adhesivewith pH about 1.5,33 and has been classified as an inter-mediately strong adhesive.41 AFM imaging of dentin sur-faces treated with AdheSE (Fig 2b) indicated that its primerpartially dissolved the smear layer and smear plugs. In theSEM evaluation (Fig 4b), a remnant of smear layer was ob-served at the resin/dentin interface, with areas that did notshow resin penetration. These results are consistent withthe previous studies.7,25,38 Remnants of smear layer with ar-eas completely devoid of resin penetration were observed inresin/dentin interface samples treated with AdheSE self-etching adhesive.7 Since contemporary self-etching adhe-sives have higher pH values than the phosphoric acid used,and are not rinsed away, the smear layer or its componentsare incorporated into the bonded layers.25 For intermedi-ately strong self-etching adhesives, the smear layer andsmear plugs were partially dissolved and remnants of smearlayer and smear plugs were observed in TEM.38

Sodium hypochlorite application prior to acid etching didnot influence the bond strength of Excite. Lai et al16 showedno effect on TBS of Excite with sodium hypochlorite treat-ment prior to the acid etching of dentin, which is consistentwith our results. Unfortunately, those authors did not explainwhy sodium hypochlorite application prior to acid etching didnot affect Excite bond strength to dentin. However, the useof sodium hypochlorite significantly reduced the bondstrength of Single bond adhesive.16,31,43 On the other hand,a significant increase in shear bond strength was reportedwith Prime & Bond 2.1 adhesive,31 which indicated that thebond strength of dentin surfaces treated with sodiumhypochlorite, prior to or after acid etching, is material de-pendent. In the current study, application of 37% phosphor-ic acid etchant completely removed the smear layer anddemineralized the underlying dentin. Complete removal ofsmear layer might eliminate the action of the protein chlo-ramine-derived radicals and subsequently prevent the pre-mature termination of adhesives free-radical polymeriza-tion. This could explain why reduction in TBS was not foundin the group treated with sodium hypochlorite prior to the ap-plication of Excite adhesive in this study.

CONCLUSION

The application of 5.25% commercial sodium hypochloritewith rubbing action for 60 s (total application time 120 s)seems to positively influence the TBS of the self-etching ad-hesive; however, it has no significant effect on the TBS of theetch-and-rinse single-bottle adhesive to dentin. The additionof functional roughness parameters to study the dentin sur-face showed to be of importance in evaluating the relation-ship between bond strength and surface topography of con-ditioned dentin. Nevertheless, further investigations areneeded.

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Clinical relevance: Based on the results of this study, pretreatment of dentin surfaces with 5.25% sodiumhypochlorite prior to the self-etching adhesive is a benefi-cial step to improve the dentin bond strength.

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